There  are  three  general  types  of  single-phase distribution  transformers.  The  conventional  type  (fig. 2-6) requires a lightning arrester and fused cutout on the primary  phase  conductor  feeding  it.  The  self-protected (SP)  type  has  a  built-in  lightning  protector.  The completely  self-protected  (CSP)  type  has  the  lightning arrester and current-overload devices connected to the transformer.  It  requires  no  separate  protective  devices. Secondary  Mains Secondary mains or circuits are the lines that carry the  electric  power  from  the  secondary  side  of  the transformer through a distribution system to supply the electrical loads. They may or may not be on the same pole with the feeder lines. If on the same pole, they may be either on a crossarm below the feeder lines or, as shown in figure 2-6, on spool racks attached to the side of the pole below the feeder lines. The secondary circuits may have several wires (service drops) connected to various buildings to serve their electrical needs. Where a large load is in demand, a transformer or transformer bank may be located at the building site. SINGLE   PHASE.—   Single-phase   secondary circuits usually supply current for electrical lighting loads, small electric appliances, and small (1 horse- power and under) single-phase electric motors. The secondaries  consist  of  two  hot  conductors  and  one neutral  conductor.  In  overhead  construction,  these conductors are mounted on the bottom crossarm on a pole or on spools attached to the side of a pole. (See fig. 2-6.)  One  transformer  will  feed  this  circuit  if  the required load to be served is not too heavy. Where the load is heavy or where several buildings are served, a bank  of  three  transformers  may  feed  the  circuit. The normal voltage of a single-phase circuit is 120 volts from either one of the energized conductors to the neutral   or   240   volts   across   the   two   energized conductors. THREE PHASE.— Some facilities, such as motor pools,  industrial  shops,  and  water  and  sewage  plants, may have equipment using three-phase motors, which require  three-phase  power.  Transformer  banks  are installed to supply this power. If a number of buildings in  the  area  require  three-phase  power,  cluster  mount may  be  installed  with  the  three-phase  secondaries extending in two or three directions and with service drops extending from the secondary to the buildings. Service Drops As  you  learned  in  the  EA3  TRAMAN,  each building requiring electric current must have lead-in conductors, known as service drops. These may be two-, three-, or four-wire conductors or a single cable containing the required number of conductors. A service drop may be connected to a secondary main to provide service  to  a  small  load.  Where  a  transformer  bank services a building requiring a large power load, the secondary  becomes  the  service  drop,  since  it  feeds current to one load only. Most Navy buildings are not metered. However, where it is desired to know how much electricity is being consumed, a meter is installed ahead of the main switch to the building. In this case, the service drop is connected to the meter before it is connected to the mains. CONTROL AND PROTECTIVE DEVICES A  power-distribution  circuit,  like  any  other electrical  circuit,  requires  the  use  of  special  devices  to provide control and to protect the system from internal or  external  influences  that  may  damage  the  circuit  or injure  personnel. Distribution Cutouts, Switches, Reclosers, and Circuit Breakers A   distribution   cutout   is   used   to   protect   the distribution  system  or  the  equipment  connected  to  it. Distribution cutouts are used with the installation of transformers (fig. 2-6), capacitors, cable circuits, and at sectionalizing points on overhead circuits. Two types of switches used in power distribution are the air switch and the oil switch. Both devices are used to connect or disconnect a portion of the power distribution  system.  The  air  switch  is  used  for  the overhead section of the distribution system, and the oil switch  is  used  with  underground  portions. Reclosers  are  for  overload  protection  and  are designed to open a circuit in an overload condition and then  automatically  reclose  the  circuit.  Reclosers  come in single-or three-phase models and can either be pole mounted or installed in a substation. Oil, air, gas, and vacuum circuit breakers are used to switch electric circuits and equipment in and out of the  system.  They  may  be  operated  manually,  by  remote control,   or   automatically   under   predetermined conditions or when electrical failures in the system occur. Lightning Arresters The purpose of installing a lightning arrester (fig. 2-6) on primary lines is twofold: first, to provide a point in the circuit at which a lightning impulse can pass to earth, through a ground wire, without injuring line insulators,  transformers,  or  other  connected  equipment; 2-5